1. Field of the Invention
The present invention relates to a gas sensor.
2. Description of the Related Art
Gas sensors that detect a predetermined concentration of a gas component, such as NOx or oxygen, contained in gas to be measured, such as exhaust gas of an automobile, have been widely known. In such gas sensors, water generated when an engine is started, for example, adheres to a sensor element and, therefore, the temperature of the sensor element decreases. Thus, cracking may occur in the sensor element. Therefore, to prevent the occurrence of cracking, it is proposed that a protective cover is attached to the sensor element so as to cover the sensor element. For example, PTL 1 describes a gas sensor including a sensor element having a protective cover on the outer periphery of the free end portion of the sensor element. The protective cover has a dual structure including a ventilation hole formed therein. The ventilation hole allows exhaust gas to enter the inside of the protective cover.